Perhaps three-quarters of the population of the United States do not know that over nearly one-half of the national territory within the United States the rainfall is so slight or so unevenly distributed that agriculture cannot be carried on except by means of irrigation. This irrigation consists of taking water out of the streams and conducting it by means of ditches which have a very gentle slope over the land which it is proposed to irrigate. From the original ditch, smaller ditches are taken out, running nearly parallel with each other, and from these laterals other ditches, still smaller, and the seepage from all these moistens a considerable area on which crops may be grown. This, very roughly, is irrigation, a subject of incalculable interest to the dwellers in the dry West. It is obvious that irrigation cannot be practiced without water, and that every ditch which takes water from a stream lessens the volume of that stream below where the ditch is taken out. It is conceivable that so many ditches might be taken out of the stream, and so much of the water lost by evaporation and seepage into the soil irrigated, that a stream which, uninterfered with, was bank full and even flowing throughout the summer, might, under such changed condition, become absolutely dry on the lower reaches of its course. And this, in fact, is what has happened with some streams in the West. Where this is the case, the farmers who live on the lower stretches of the stream, being without water to put on their land, can raise no crops. Nothing, therefore, is more important to the agriculturists of the West than to preserve full and as nearly equal as possible at all seasons the water supply in their streams. This water is supplied by the annual rain or snow fall; but in the West chiefly by snow. It falls deep on the high mountains, and, protected there by the pine forests, accumulates all through the winter, and in spring slowly melts. The deep layer of half-rotted pine needles, branches, decayed wood and other vegetable matter which forms the forest floor, receives this melting snow and holds much of it for a time, while the surplus runs off over the surface of the ground, and by a thousand tiny rivulets at last reaches some main stream which carries it toward the sea. In the deep forest, however, the melting of this snow is very gradual, and the water is given forth slowly and gradually to the stream, and does not cause great floods. Moreover, the large portion of it which is held by the humus, or forest floor, drains off still more gradually and keeps the springs and sources of the brook full all through the summer.